The present invention relates to an extruder head for fabricating a helicoidal element of synthetic material, comprising a rotary die provided with an axial orifice whose form corresponds to the transverse profile of the element to be fabricated, a mandrel rod aligned with the die, an intermediate rotary sleeve, which is coupled to a rotary driving mechanism and inside which the die and the mandrel rod are centrally fixed, this sleeve being provided with channels for passage of the synthetic material and a distributing cartridge fixedly mounted and provided with an axial bore, in which the intermediate sleeve is accomodated, and with distribution channels for distributing the synthetic material on the periphery of this sleeve.
According to another aspect, the invention also relates to an extruder head for fabricating an element of synthetic material of a first color having at least one helicoidal band of synthetic material of a second color, this head comprising a rotary die provided with an axial orifice, whose form corresponds to the transverse profile of the element and with at least one transversal feed channel for the material of the second color, a mandrel rod aligned with the die, an intermediate rotary sleeve, which is coupled to a rotary driving mechanism and inside which the die and the mandrel rod are centrally fixed, this sleeve being provided with cross-channels for passage of the synthetic material and a distributing cartridge fixedly mounted and provided with an axial bore, in which the intermediate sleeve is accomodated, and with distribution channels for distributing the synthetic material on the periphery of the sleeve.
The invention also relates to a process for lubricating an extruder head with a rotary die.
Extruder heads wtih a rotary die are utilized to produce elements whose outer surface has a helicoidal form, especially sheaths formed on the central conductor of a coaxial cable and provided with a helicoidal cheek serving to center this conductor with respect to the outer conductor, or cylindrical rods with helicoidal grooves serving as a support for optical fibers, in an optical cable. The extruder heads wtih a rotary die which are currently known present several problems which can only be solved by complicated and costly means. As the synthetic material delivered under pressure by the extruder reaches the die through a conical channel defined on one side by a fixed mandrel rod, and on the other by the rotary die, the die is subjected to a very strong axial thrust, which must be taken up by a thrust ball or roller bearing which is costly and bulky. The European patent application published under No. 0078 213 illustrates an example of such an arrangement.
On the other hand, it is difficult to obtain proper tightness between the rotary die and its fixed support in the extruder head, so as to avoid that leaks of synthetic materials foul up the ball bearing or bearings. It is generally necessary to provide labyrinth joints, which are costly and delicate.
Lastly, the presence of these joints and the ball or roller bearings makes heating of the die more difficult, particularly heat transmission by conduction.
For covering a wire by means of two insulating materials of different colors forming helicoidal bands, the patent CH-A-258422 provides a rotary wire guide on which the die is screwed and which encloses channels for distributing synthetic materials from two annular grooves provided on its peripheral surface. The wire guide revolves in an axial bore of a distributing cartridge which is fixed. This system absolutely does not permit precise centering of the die with respect to the wire guide. In addition, the structure of the wire guide, with the distribution channels and the driving mechanism which it comprises is extremely complicated and delicate. Lastly, nothing is provided to resolve the problems of tightness between the mandrel rod and the distributing cartridge.
The U.S. Pat. No. 2,779,790 also provides for a head for extruding a two-tone element having helicoidal bands. The structure described in this document is of the type indicated in the preamble. The sliding and bearing surface of the intermediate rotary sleeve on the fixed distributing cartridge is conical, the sleeve being pressed axially against the cartridge by means of a spring which bears upon the rotary driving mechanism of the sleeve. In addition to the axial forces exerted on the body of the head and on the driving mechanism due to the thrust of the spring, this structure is unfavorable with regard to tightness along the sliding surface because the pressure of the material at this conical surface must be balanced by the thrust of the spring. Now, this pressure may be very high (up to 700 bars or more) and it may vary substantially in the course of a given fabrication.